Single-ended-to-differential converter with common-mode voltage control

ABSTRACT

Provided is a circuit to perform single-ended to differential conversion while providing common-mode voltage control. The circuit includes a converter to convert a single-ended signal to a differential signal and a stabilizing circuit adapted to receive the differential signal. The stabilizing circuit includes a sensor configured to sense a common-mode voltage level of the differential signal and a comparator having an output port coupled to the converter. The comparator is configured to compare the differential signal common-mode voltage level with a reference signal common-mode voltage level and produce an adjusting signal based upon the comparison. The adjusting signal is applied to the converter via the output port and is operative to adjust a subsequent common-mode voltage level of the differential signal.

This application is a continuation of the U.S. Non-ProvisionalApplication entitled “Single-Ended-To-Differential Converter withCommon-Mode Voltage Control,” Ser. No. 10/425,736, filed Apr. 30, 2003,now U.S. Pat. No. 6,727,756 which is a continuation of U.S.Non-Provisional Application entitled “Single-Ended-To-DifferentialConverter with Common-Mode Voltage Control,” Ser. No. 10/105,253, filedMar. 26, 2002, now U.S. Pat. No. 6,771,127, all of which is incorporatedherein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to single-ended to differentialconverters. More particularly, the present invention relates to acircuit technique to perform attenuation and single-ended todifferential conversion with common-mode voltage control on a singleintegrated circuit (IC).

2. Background Art

Advances in electronic device packaging provide electronic systemdesigners with the ability to include an increased number of functionson a single IC. Such ICs are particularly well suited for low supplyvoltage applications, especially those ICs that can accommodateprocessing of analog signals. Thus, by combining numerous functions on asingle IC, electronic system designers are able to realize tremendoussavings in power consumption and space. Even greater functionaladvantages can be realized through the selection of particular signalprocessing techniques to be used on these ICs.

For example, in the case of analog signals on single ICs, differentialsignal processing is preferred over single-ended processing because itprovides better rejection of power supply and substrate noises.Differential signal processing is also inherently better at rejectingcommon-mode voltages than single-ended processing. For purposes ofillustration, differential signal processing is commonly used insingle-chip video receivers, also known as on-chip receivers. In on-chipreceivers, however, certain applications often benefit more from theinput of single-ended signals instead of differential signals. Also,from the standpoint of hardware, single-ended signals may be preferredas circuit inputs because they are easier to provide as circuit inputsand can be more cheaply produced than their differential-ended versions.When single-ended signals are provided as circuit inputs, conventionalon-chip receivers normally include additional on-chip circuitry toconvert the received single-ended signals into differential signals inorder to facilitate the more advantageous differential signalprocessing.

A number of traditional techniques exist to convert single-ended signalsto differential signals. One popular technique includes converting thesingle-ended signal from the voltage domain to the current domain. Withthis particular technique, the single-ended signal is received at one ofthe inputs of a voltage to current converter. In response, the converterproduces at its output, a differential signal, having a positive signalcomponent and a negative signal component. Both signal components,however, are in the current domain. To convert this differential currentsignal from current domain to voltage domain, the differential signal isforwarded to a current to voltage converter, such as resistor, in orderto finally create the differential voltage signal. One deficiency withthis approach is that it's not well suited for low voltage powersupplies. More specifically, it's difficult to fit this approach withinthe voltage head-room constraints of low voltage power supplies.

Another technique for converting single-ended signals to differentialsignals includes the use of a degenerate differential pair. However,when providing one side of a degenerate differential pair with arelatively large signal, while at the same time, leaving the other sideat a constant voltage, its extremely difficult to achieve acceptablelinearity. Providing one side of the degenerate differential pair with alarge signal and leaving the other side at a constant voltage isrequired in order to convert a single-ended signal to a differentialsignal.

In addition to single-ended to differential converters, attenuators arealso circuits that are commonly used in on-chip receivers. When used toattenuate input signals, the dynamic range of attenuators can beadjusted to fit within the dynamic range of subsequent signal processingblocks, such as track and hold circuits or analog-to-digital converters(ADCs).

Although numerous traditional techniques exist to convert single-endedsignals into differential signals, low voltage power supplies often donot provide adequate voltage head-room to accommodate efficient use ofthese traditional techniques. As a result, there is a need for a devicethat can provide and improved approach for converting a single-endedsignal to a differential signal without the deficiencies of thetraditional approaches discussed above. Also, there is a need to providean improved single-ended to differential conversion technique and anattenuator on a single IC chip to save power and optimize circuit boardspace.

BRIEF SUMMARY OF THE INVENTION

Consistent with the principles of the present invention as embodied andbroadly described herein, an exemplary circuit includes an attenuatorhaving first and second receiving ports configured to respectivelyreceive first and second input signals and differential output portsconfigured to output a differential signal produced in accordance withthe first and second input signals. The attenuator also includes anadjusting node positioned in association with the differential outputports. A stabilizing circuit, including a sensor and a comparator, isconfigured to receive the differential signal. The sensor is coupledacross the differential output ports and has a feedback node. Thecomparator includes two input ports and a comparator output port. Afirst of the two input ports is coupled to the feedback node and theother of the two input ports is configured to receive a referencevoltage signal. The comparator output port is coupled to the adjustingnode. The sensor is configured to detect a common-mode voltage level ofthe differential signal and provide the detected common-mode voltagelevel to the first input port. The comparator is configured to comparethe common-mode voltage level to a level of the reference voltage signaland produce an adjusting voltage signal based upon the comparison.Finally, the adjusting voltage signal is applied to the adjusting nodeand is operative to adjust a subsequent common-mode voltage level of thedifferential signal.

Features and advantages of the present invention include the ability tobeneficially combine the functions of a low power single-ended todifferential converter and an attenuator on a single IC. This approachincreases the performance and provides additional space on the IC forincorporation of added functions. Additional advantages of the presentinvention are the ability to provide single-ended to differentialconversion in a manner that fits within the voltage head-roomconstraints of low voltage power supplies and a provision forcontrolling common-mode voltage in an efficient manner.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the inventionand, together with the description, explain the purpose, advantages, andprinciples of the invention. In the drawings:

FIG. 1 is a block diagram of an exemplary implementation of the presentinvention;

FIG. 2 is a schematic diagram of an exemplary circuit constructed andarranged in accordance with the present invention;

FIG. 3 is a schematic diagram of the circuit of FIG. 2 including voltagebuffers;

FIG. 4 is a schematic diagram of a conventional amplifier used in thecircuits of FIG. 2 and FIG. 3;

FIG. 5 is a block diagram of a programmable gain attenuator used in apreferred embodiment of the present invention; and

FIG. 6 is a flow chart of an exemplary method of practicing the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware, software, firmware, and/or the entitiesillustrated in the figures. Any actual software code with specializedcontrolled hardware to implement the present invention is not limitingof the present invention. Thus, the operation and behavior of thepresent invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 is a block diagram of the exemplary embodiment of the presentinvention. Included in FIG. 1 is a circuit 100 including a converter 102and a stabilizing circuit 106. The converter 102 is provided to converta single-ended input single V_(sig) received at an input terminal 101,into a differential output signal V_(out). The differential outputsignal V_(out) is then provided at respective positive and negativedifferential output terminals 104 and 105. In order to make theconverter 102 symmetrical, an input terminal 103 is provided forreceiving a matching DC voltage signal V_(dc) as an input. That is, thesignal V_(dc), provided from a DC voltage source, has a voltage levelsubstantially equivalent to an average of the voltage level of signalV_(sig). The converter 102, as will be discussed in greater detailbelow, receives the input signals V_(sig) and V_(dc) and converts thesesignals into the differential output signal V_(out). The symmetrybetween the relationship of V_(sig) and V_(dc) minimizes substrate noisesince signals having roughly equivalent average signal levels are inputat both sides of the converter 102. In an exemplary embodiment, thedegree to which the levels between V_(sig) and V_(dc) match is withinabout ±10%, although other tolerances may be selected. This symmetryprovides a degree of balance to the converter and prevents theoccurrence of substrate and other noises appearing at the outputterminals 104 and 105.

As stated above, a popular technique for converting single-ended signalsto differential signals includes converting the received single-endedsignal from the voltage domain to current domain and then back to thevoltage domain. Although this technique is not particularly well suitedfor low voltage application, an inherent benefit is elimination ofundesirable common-mode voltages. As known in the art, common-modevoltages may result from a number of different factors such as impropercircuit grounding or noise in the actual input source, etc. Convertingsingle-ended signals to differential signals using the voltage tocurrent conversion technique removes these unwanted common-mode voltagecomponents. In the present invention, however, the stabilizing circuit106 is provided to remove unwanted common-mode voltage components, inplace of the voltage to current conversion technique discussed above.

The stabilizing circuit 106 includes a sensor 107 and a comparator 108.The sensor 107 is configured to detect the common-mode voltage level ofthe differential output signal V_(out). In particular, the sensor 107 iscoupled to the converter 102 via connection leads 109 and receives thedifferential signal V_(out) as an input thereto. The sensor 107 thenprovides a measurement of the common-mode voltage level of thedifferential output signal V_(out) to the comparator 108. The comparator108 also receives a reference common-mode voltage signal V_(cm) as aninput from a reference voltage source (not shown). The comparator 108compares the measured common-mode voltage level of the differentialsignal V_(out) with the reference common-mode voltage signal V_(cm) andprovides an adjusting signal V_(adj) at an output terminal 110 of thecomparator 108. The adjusting signal is representative of the differencebetween the reference common-mode voltage signal V_(cm) and thecommon-mode voltage level of the differential output signal V_(out). Theadjusting signal V_(adj) is then provided to the converter 102 along afeedback path 111 in order to adjust the common-mode voltage level ofthe differential output signal V_(out).

The output of the comparator 108 may be used in a number of differentways. The value of the adjusting signal V_(adj) output from thecomparator 108 is equal to a gain (A) of the comparator 108 multipliedby a difference between the voltage V_(cm) and the common-mode voltagelevel of the differential output signal V_(out). In other wordsV_(adj)=A*(V_(cm)−V_(out)). The adjusting signal V_(adj) is thereforeoperative to adjust the common-mode voltage level of the differentialsignal V_(out), more specifically, a subsequent common-mode voltagelevel of V_(out).

FIG. 2 provides a more detailed view of the circuit 100, including theconverter 102 and the stabilizing circuit 106. The converter 102includes an attenuator formed of two sets of impedance devices 206 a/206b and 208 a/208 b. The primary function of the impedance devices 206a/206 b and 208 a/206 b is to attenuate V_(sig) and convert V_(sig) to adifferential output signal. As shown in FIG. 2, the impedance devices208 a and 208 b are positioned adjacent to one another and connectedtogether in series. The impedance device 206 a is connected in serieswith the impedance device 208 a and the impedance device 206 b isconnected in series with the impedance device 208 b.

In a preferred embodiment of the present invention, the impedancedevices 206 a and 206 b have a substantially equal impedance value (Z₁),and the impedance devices 208 a and 208 b have a substantially equalimpedance value, (Z₂). Furthermore, the value of impedance devices 206 aand 206 b is chosen as a function of the desired degree of attenuationto be provided by the converter 102. A connection node between theimpedance devices 206 a and 208 a forms the output terminal 104 and aconnection node between the impedance devices 206 b and 208 b forms theoutput terminal 105. The signal input terminal 101 is formed by anunconnected side of the impedance device 206 a. Additionally, the signalinput terminal 103 is formed by an unconnected side of the impedancedevice 206 b.

As explained above, during operation of the circuit 100, thesingle-ended input signal V_(sig) is provided to the input terminal 101and the DC voltage signal V_(dc) is provided at the input terminal 103.Although the differential signal V_(out) may also be produced as aresult of applying only the input signal V_(sig) to the input terminal101, the input signal V_(dc) is provided at the input terminal 103 tomake the converter 102 symmetrical, as discussed above. When V_(sig) andV_(dc) are applied to respective input terminals 101 and 103 in a nearsimultaneous manner, the differential output signal V_(out) is producedat the output terminals 104 and 105.

The conversion technique used by the converter 102 provides asignificant improvement in linearity over the conventional approaches.For example, in video applications, a linearity of about 60 dB or betteris desirable to insure adequate processing of video signals. However,using the degenerate differential pair technique of the conventionalapproaches, the achieved linearity may be much less than 60 dB. In anexemplary embodiment of the present invention, however, linearities ofabout 80 dB are possible, over 20 dB better than the conventionalapproaches. Although the conversion technique of the present inventionprovides better linearity, it lacks the inherent common-mode rejectioncapability inherent in the conventional approaches. Therefore, thestabilizing circuit 106 is provided to suppress the common-mode voltageof the differential signal V_(out).

The common-mode sensor 107 of the stabilizing circuit 106 includes athird set of impedance devices 210 a and 210 b, also connected inseries. In a preferred embodiment of the present invention, theimpedance devices may be resistors or any other suitable devices. Also,the impedance devices 210 a and 210 b have a substantially equalimpedance value Z_(sense). Further, in most applications, the impedancevalue of the impedance devices 210 a and 210 b is higher than theimpedance value of the devices 208 a and 208 b. One end of the device210 a is coupled to the output terminal 104, and one end of theimpedance device 210 b is coupled to the output terminal 105. Connectedin this manner, the impedance devices 210 a and 210 b are arranged tomeasure the common-mode voltage of the differential output signalV_(out).

Also as shown in FIG. 2, a connection node 213 is formed between theimpedance devices 210 a and 210 b and is coupled to an inverting inputterminal 216 of the comparator 108. The comparator 108 may be forexample, an operational amplifier. The reference common-mode voltageV_(cm) is provided as an input to a non-inverting input terminal 214.The reference voltage V_(cm) may be provided by any conventional voltagegeneration means. Finally, the output terminal 110 is connected via thefeedback path 111, to a node 218 between the impedance devices 208 a and208 b, thus providing a negative feedback mechanism. The comparator 108receives the detected common-mode voltage from the sensor 107, comparesit with the reference common-mode voltage V_(cm), and adjusts thedetected common-mode voltage to match V_(cm). That is, the comparator108 provides a push pull arrangement which suppresses the common-modevoltage of the differential output signal V_(out), if its common-modevoltage is greater than V_(cm), and boosts the common-mode voltage ifthe common mode voltage is less than V_(cm).

Since most circuits that receive outputs from single-ended todifferential converters, such as track and hold circuits and ADCs, aresensitive to common-mode jump, an efficient technique is desirable forcontrolling the common-mode voltages associated with the differentialsignal V_(out). In the embodiment shown in FIG. 2, the adjusting signal,provided at the output terminal 110, is injected into the converter atthe node 218 in order to adjust the common-mode output of thedifferential output signal V_(out). The common-mode voltage associatedwith V_(out) can be adjusted up or down in accordance with the techniquediscussed above.

If the voltage values of V_(sig) and V_(dc) are suitable to drive theentire circuit 100, they may be applied directly to input terminals 101and 103, as shown in FIG. 2. If, on the other hand, V_(sig) and V_(dc)are not suitable for driving the entire circuit 100 then voltage buffersmay be required at the input terminals 101 and 103 as shown in FIG. 3,to provide increased driving capability.

FIG. 3 is an illustration of the circuit arrangement of FIG. 2 modifiedto include voltage buffers 300 and 304. The voltage buffers 300 and 304are respectively connected to the input terminals 101 and 103. Thebuffers 300 and 304 may be formed, for example, of operationalamplifiers. The voltage buffer 300 includes a non-inverting inputterminal 302 adapted to receive the input signal V_(sig) as an input andan inverting input terminal 303 coupled to a buffer output terminal 305.The output terminal 305 is connected to the input terminal 101 of theconverter 102. Similarly, the voltage buffer 304 includes anon-inverting input terminal 306 adapted to receive the input signalV_(dc) as an input and an inverting input terminal 308 connected to abuffer output terminal 309. The output terminal 309 is connected to theinput terminal 103 of the converter 102. In this arrangement, theattenuator formed by the impedance devices 206 a, 206 b, 208 a, and 208b loads the outputs of the buffers 300 and 304. The buffers thenattenuate V_(sig) in accordance with the following expression:$\frac{Z_{2}{\quad}Z_{sense}}{Z_{1} + {Z_{2}{\quad}Z_{sense}}}$

The differential output voltage V_(out) is the output of the converter102. As stated above however, if the signals V_(sig) and V_(dc) areadequate to drive at the circuit 100, then the buffers 300 and 304 willnot be needed.

The suitability of the converter 102, shown in FIGS. 2 and 3, foroperation in a low voltage environment depends on the implementation ofthe amplifiers used in the comparator 108 and the voltage buffers 300and 304. FIG. 4 shows an exemplary well known op-amp implementation thatis suitable for this application. The op-amp has respective invertingand non-inverting input terminals labeled (−in) and (+ip) and an outputterminal labeled (out).

Each circuit shown in FIGS. 2 and 3 is substantially internallysymmetrical except that each is driven in a single-ended manner.Symmetry is beneficial for rejection of power supply and substratenoise, with the symmetry being defined as the ability to swap V_(sig)and V_(dc), and the use of virtually identical circuit components oneach side of the converter 102 as shown, for example, in FIGS. 2 and 3.In the case of FIG. 2 and FIG. 3, although the converter 102 isconfigured as a single-ended to differential converter, it can also bedriven differentially by applying a differential signal to the voltagebuffers 300 and 304.

An exemplary embodiment of the present invention can also be configuredto accommodate programmability. For example, FIG. 5 shows a conventionalprogrammable gain attenuator 500, which can be directly connected to theoutput terminals 104 and 105 of the single-ended to differentialconverter to permit a user to program specific attenuation valuesrelated to the output signal V_(out).

FIG. 6 illustrates a flowchart of an exemplary method of practicing thepresent invention. As shown in FIG. 6 and explained with reference toFIG. 2, the conversion process begins by the circuit 100 receiving asingle-ended signal at the input terminal 101 and a DC signal at theinput terminal 103 as described in block 600 of FIG. 6. Next, thesingle-ended signal is attenuated and a differential signal is producedat the output terminals 104 and 105 as described in block 602.

The sensor 107 measures the common-mode voltage level of thedifferential signal and provides the detected common-mode voltage levelto the inverting input terminal 216 of the comparator 108 as describedin blocks 604 and 606 respectively. Next, the reference signal V_(cm) isprovided at the non-inverting input terminal the comparator 108, and iscompared with the detected common-mode voltage level of the differentialsignal as described in blocks 608 and 610 respectively. Finally, anadjusting signal is provided at the output terminal 110 of thecomparator 108 along the path 111. The adjusting signal is injected intothe converter 102 in order to adjust the subsequent common-mode voltagelevel of the differential signal based upon the adjusting signal asrespectively described in blocks 612, 614, and 616.

CONCLUSION

Using the technique of the present invention, a single-ended signal canbe converted into a differential signal in a low voltage environmenthaving significantly improved linearity characteristics when compared toconventional techniques. Also, the technique of the present inventioncan be used to adjust and suppress unwanted common-mode voltages in astable manner. Finally the functions of single ended to differentialconversion, attenuation, and common-mode voltage adjustment can becombined and provided on a single IC to provide hardware savings onassociated circuit boards.

The foregoing description of the preferred embodiments provide anillustration and description, but is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Modification andvariations are possible, consistent with the above teachings or may beacquired from practice of the invention. Thus, it is noted that thescope of the invention is defined by the claims and their equivalents.

1. A method to convert a single-ended signal to a differential signal,comprising: attenuating a single-ended signal received within anattenuator and producing a differential signal at differential outputports formed within the attenuator, the differential signal being basedupon the attenuated single-ended signal and a received DC signal;detecting a common-mode voltage level of the differential signal andproviding the detected common-mode voltage level to an inverting inputport of an amplifier; providing a reference common-mode voltage signalto a non-inverting input port of the amplifier; comparing the detectedcommon-mode voltage level with a voltage level of the reference signaland providing an adjusting signal at an amplifier output port based uponthe comparison; and adjusting common-mode voltage levels of subsequentlyproduced differential signals based upon the adjusting signal.
 2. Themethod of claim 1, wherein an adjusted common-mode voltage level issubstantially equal to the voltage level of the reference signal.
 3. Anapparatus for converting a single-ended signal to a differential signal,comprising: means for attenuating a single-ended signal received withinan attenuator and producing a differential signal at differential outputports formed within the attenuator, the differential signal being basedupon the attenuated single-ended signal and a received DC signal; meansfor detecting a common-mode voltage level of the differential signal andproviding the detected common-mode voltage level to an inverting inputport of an amplifier; means for providing a reference common-modevoltage signal to a non-inverting input port of the amplifier; means forcomparing the detected common-mode voltage level with a voltage level ofthe reference signal and providing an adjusting signal at an amplifieroutput port based upon the comparison; and means for adjustingcommon-mode voltage levels of subsequently produced differential signalsbased upon the adjusting signal.
 4. The apparatus of claim 3, wherein anadjusted common-mode voltage level is substantially equal to the voltagelevel of the reference signal.